Galaxy Number Counts

In these Lectures Notes I re-visit the problem of the deep number counts as a viable method to (1) study the evolution of the galaxy population in the past and (2) determine the value of the deceleration parameter $q_0$. After a brief description of the basic concepts needed to model the counts, we show that simple luminosity evolution models which incorporate dust are able to provide reasonable fits to both the deep counts and the redshift distribution of faint field galaxies if $q_0=0.05$. In order to fit the counts in a {\it closed} ($q_0=0.5$) Universe, we include a population of fading dwarfs, more numerous in the past, based on current ideas to understand the nature of dwarf galaxies.